There is no requirement for you to go to the newest libsoup gtk3 libs, except maybe some methods may be changed or missing. You could go back and get the libsoup that was compatible with your current gtk2 libs. Another option may be webkit. A Perl module exists to use it, but I don't know if it would handle your authentication requirements.

You can go back into the older directories at gnome sources, to find your compatible version. It looks like libsoup goes way back into gtk2 versions before Glib::Object::Introspection even showed up on the scene.

The switch from gtk2 to gtk3 occurred at GTK+ 2.24. So I would start with the libsoup with the ftp date closest to 2.24. Libs after that point are still gtk3 although they have a 2.xx number.